Electron spectrometer

ABSTRACT

An electron spectrometer comprising an electron source for providing a beam of electrons having a predetermined energy spectrum, and an electron selector for dispersing in energy the electrons of the beam to produce an elongate selector image, each portion of the length of which includes electrons having a predetermined respective energy. The selector image is focussed on a target to produce an elongate target image including scattered electrons having a range of energies, each portion of the length of the target image resulting from a respective portion of the length of the selector image. An analyzer disperses in energy the scattered electrons of the target image, the analyzer being orientated such that the electrons are dispersed in a direction substantially perpendicular to the length of the target image, whereby the analyzer produces a rectangular image made up of substantially parallel strips, each including electrons of a range of energies but each resulting from the scattering by the target of electrons of a respective energy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electron spectrometer of the type inwhich a substance under investigation is bombarded with electrons andthe spectra of electrons scattered from the substance are studied.Spectrometers of this type are used to investigate for example theatomic and molecular structure of materials. The substance which isbombarded with electrons is generally referred to as the target.

The basic principles of electron spectrometry are well known and auseful description of these principles is contained in U.S. Pat. No.3,777,159. In conventional electron spectrometers, a beam of electronsprovided by an electron source is initially dispersed in energy. Amonochromatic beam (in which all the electrons have the same energy) isselected from the dispersed output of the electron source and themonochromatic beam is used to bombard a target. A beam of scatteredelectrons results the energies of which vary and this beam of scatteredelectrons is in turn dispersed in energy and detected by a linear arrayof detectors. The output of each detector is representative of thenumber of electrons scattered from the target having an energycorresponding to the position of that detector.

It has been proposed to provide two dimensional electron spectrometersin which the single linear array of detectors provided in onedimensional spectrometers is replaced by a two dimensional array made ofa series of arrays of detectors arranged in parallel. In one knownelectron spectrometer the detectors are in the form of a 100×100photodiode array. In all the known electron spectrometers however, thetarget is bombarded with a monochromatic beam of electrons so thatappropriate conclusions can be drawn from the detected energy of theresultant scattered electrons. The use of a monochromatic beam doeshowever mean that only a small proportion of the output of the electronsource is in fact utilized to bombard the target.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the sensitivity ofelectron spectrometers.

According to the present invention there is provided an electronspectrometer comprising an electron source for providing a beam ofelectrons having a predetermined energy spectrum, an electron selectorfor dispersing in energy the electrons of the beam to produce anelongate selector image each portion of the length of which compriseselectrons having a predetermined respective energy, means for focussingthe selector image on a target to produce an elongate target imageincluding scattered electrons having a range of energies, each portionof the length of the target image resulting from a respective portion ofthe length of the selector image, and an analyzer for dispersing inenergy the scattered electrons of the target image, the analyzer beingorientated such that the electrons are dispersed in a directionsubstantially perpendicular to the length of the target image, wherebythe analyzer produces a rectangular image made up of substantiallyparallel strips each including electrons of a range of energies but eachresulting from the scattering by the target of electrons of a respectiveenergy.

The invention also provides a method for producing a two-dimensionalimage of a target in an electron spectrometer, wherein a beam ofelectrons having a range of energies is dispersed in energy to producean elongate selector image each portion of the length of which compriseselectrons having a predetermined respective energy, the selector imageis focussed on the target to produce an elongate target image includingscattered electrons having a range of energies, each portion of thelength of the target image resulting from a respective portion of thelength of the selector image, and the scattered electrons of the targetimage are dispersed in energy, the direction of dispersion of thescattered electrons being substantially perpendicular to the length ofthe elongate target image, whereby a rectangular image is formed ofsubstantially parallel strips each including electrons of a range ofenergies but each resulting from the scattering by the target ofelectrons of a respective energy.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a known multidetectorspectrometer;

FIG. 2 is a schematic illustration of the paths followed by electrons inan embodiment of the present invention;

FIG. 3 is a schematic illustration of component parts of the electronlenses of an embodiment of the present invention; and

FIG. 4 is a schematic illustration of components shown in FIG. 3 forrotating an electron beam.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, an electron gun 1 produces a beam of electronshaving a range of energies and this electron beam is dispersed in energyby a conventional hemispherical deflector 2. A monochromatic beam ofelectrons is selected from the output of the deflector 2 using aconventional slit aperture (not shown) and this monochromatic beam isfocussed by a first set of electron lenses 3 on a target 4. Electronsscattered from the target 4 are focussed by a second set of electronlenses 5, the scattered electrons having a range of energies.

The beam of scattered electrons is dispersed in energy using a furtherhemispherical deflector 6 so as to project an extended electron imageonto multichannel plates 7 which are of conventional form and enable theextended electron image to be amplified. The amplified electron image isaccelerated onto a phosphor screen 8 to produce an equivalent opticalimage which is then focussed by an optical system represented by lens 9onto a charge coupled device 10. The charge coupled device 10 isconnected to a conventional CCD drive 11 which supplies data to aconventional data system 12.

The extended electron image is elongate and each portion of its lengthcorresponds to a respective scattered electron energy. The energy ofelectrons incident upon the target is known and accordingly an analysisof the energy of the scattered electrons enables the structure of thetarget to be investigated.

Referring now to FIG. 2, this is a schematic illustration of the pathfollowed by electrons in a spectrometer in accordance with the presentinvention. Electrons having a range of energies originate at point 13and are dispersed in energy by a conventional hemispherical deflector orselector so as to form an elongate selector image at the positionindicated by dotted line 14, the energies being spread in the directionof arrow 15. Electrons from the selector image travel in the directionof arrow 16 and are scattered in all directions by a target. Scatteredelectrons which travel in the direction of arrow 17 result in a targetimage at the position indicated by dotted line 18. The shaded area 19corresponds to scattered electrons having a range of energies butresulting from a monochromatic sub-portion of the selector image.

Electrons from the target image are dispersed in energy in a furtherconventional hemispherical deflector or analyzer, the direction ofdispersion being perpendicular to the length of the target image. Thusthe scattered electrons from the area 19 of the target image aredispersed in the direction of arrow 20 to form the strip-shaped area 21.The area 21 is effectively an energy loss spectrum corresponding to thearea 19 of the target image. Each portion of the target image 18 will ofcourse result in a corresponding strip shaped area with the strip shapedareas making up a rectangular final image. That final image can beapplied to multi-channel plates of the type shown in FIG. 1 to produce acorresponding optical image for viewing by a two dimensional rectangularCCD device or photodiode array for example. The spectrometer thusproduces at its exit a two dimensional image containing significantinformation in both dimensions. Although there is a range of energies inthe dispersed electron beam which is used to bombard the target this hasno effect on the overall resolution. The increase in intensity of theincident electron beam results in a substantial improvement insensitivity.

The direction of dispersion in the analyzer deflector will not normallybe the same across the full width of the analyzer deflector because ofdistortion of the electron beam. Generally the direction of dispersionin the analyzer deflector is arranged to be perpendicular to the axis ofthe target image at the center of the image. On either side of thecenter of the image the direction of dispersion is only substantiallyperpendicular to the target image axis. This distortion is generallynegligible but in some circumstances appropriate corrections may bethought necessary.

Referring now to FIG. 3, the structure of electron lenses positionedbetween a selector image 22 and a target image 23 is illustrated. Theimages 22 and 23 correspond to the images appearing at the positions 14and 18 of FIG. 2. Conventional electron lens structures are used, butthe selector and analyzer are mounted at 90° to each other as comparedwith conventional spectrometers.

Referring to FIG. 3 in detail, the selector image 22 is the output of aconventional hemispherical deflector which disperses in energy theoutput of an electron gun, the dispersion being in the direction of thearrows at position 22. A first lens 24 focusses electrons from theselector image 22 on a plate 25 in which an elongate slit 26 is formed.The slit is elongate in the direction of the arrows indicated atposition 26 and thus electrons having a range of energies pass throughthe defining aperture formed by the plate 25.

Electrons passing through the slit in the plate 25 pass between a pairof plates 27 which as described in more detail hereinafter cause theelectron beam to be rotated about the axis of the lens system tocompensate for small misalignments of the selector deflector andanalyzer deflector and to compensate for the effects of any strayelectric or magnetic fields which may rotate the image slightly. Theelectron beam is then focussed by a second lens 28 onto a target at theposition indicated by arrows 29. The electron beam electrons reachingthe target position 29 are dispersed in energy in the direction of thearrows at position 29.

The image at the position indicated by arrows 29 is focussed by a thirdlens 30 onto the position indicated by the arrows 23. Thus the targetimage at the position indicated by arrows 23 corresponds to the imageshown at position 18 in FIG. 2. Each portion of the target imagecomprises electrons of a range of energies but with each of theelectrons resulting from scattering of electrons of a single energy. Theimage at position 23 is dispersed in a conventional analyser (not shownin FIG. 3) in a direction perpendicular to the plane of FIG. 3 toprovide the two-dimensional image as indicated schematically in FIG. 2.

Each of the three lenses 24, 28 and 30 consists of a series of metalcylinders to which predetermined potentials are applied to adjust thefocussing of the electron beams. Such lenses are in general use andtheir operating parameters such as image distances and focal lengths maybe obtained from standard lens tables to provide desired performancecharacteristics. For example, increasing the length of the image and thesolid angle of acceptance both increase the sensitivity but also degradethe spot size and therefore the energy resolution. The final design willtherefore be a compromise between these various factors. Some chromaticaberration is inevitable in each of the lenses as each lens is seekingto focus electrons having a range of energies.

The lenses also incorporate deflectors 31, 32, 33 and 34 which steer theelectron beams in order to correct for stray fields and misalignments.These deflectors are conventional and consist of two metal platesequi-spaced about the beam with voltages Vo±ΔV applied to give atransverse electric field with mean potential Vo. Thus the deflectorscause no rotation of the image. The resistive plates 27 do causerotation however as will be described with reference to FIG. 4.

Referring to FIG. 4, the resistive plates 27 are equi-spaced around theelectron beam 35, the intended orientation of the electron beam 35relative to the plates 27 being illustrated in FIG. 4 as the electronbeam enters the space between the two plates 27. Equal and oppositecurrents I are passed through the two plates 27 so that the potential atthe mid-point between the plates is Vo but the field varies from 0 atthe mid-point to +E and -E at the two extreme ends of the image. Thishas the effect of twisting the image and by adjusting the magnitude anddirection of the current the magnitude and direction of the angle oftwist can be adjusted to provide the desired rotation.

The data contained within the final image may be processed in anyconvenient manner. Preferably as described above the final image isanalysed by a two dimensional array, for example a charge coupled deviceor a photodiode array. Some simplification of the processing of the datacan be introduced however. For example if a small range of impactenergies is not significant to a particular investigation then spectraproduced in individual columns in the analyser array (for example thecolumn analyzing the area 21 of FIG. 2) can be added with appropriateshifts to produce one spectrum with an increased signal rate.Alternatively, when measuring excitation functions it is possible tomake use of data obtained from different columns of the analyser arrayto measure the intensities for a number of transitions over a range ofdifferent energies simultaneously.

Implementation of the present invention does raise certain potentialproblems as compared with the monochromatic beam systems of the priorart. Examples of such problems are:

A. Chromatic aberration.

B. Off-axis aberration.

C. Changes in dispersion of the image at the target.

D. Increased rate of data production.

Chromatic aberration results from the range of electron energies in theelectron beams passing through the lenses. For high resolutionmeasurements however the range of energies is very small and accordinglychromatic aberration is not a major problem.

Off-axis aberrations result from the use of an extended image. Off-axisaberration can be maintained within reasonable limits over the whole ofthe energy ranges to be considered by using conventional lens designtechniques.

Changes in dispersion of the image at the target can be minimized by theuse of constant magnification electron lenses.

The increase in the rate at which data is produced can be accommodatedby suitable modifications to the data processing circuitry.

I claim:
 1. An electron spectrometer comprising:an electron source forproviding a beam of electrons having a predetermined energy spectrum, anelectron selector for dispersing in energy the electrons of the beam toproduce an elongate selector image each portion of the length of whichcomprises electrons having a predetermined respective energy, means forfocussing the selector image on a target to produce an elongate targetimage including scattered electrons having a range of energies, eachportion of the length of the target image resulting from a respectiveportion of the length of the selector image, and an analyzer fordispersing in energy the scattered electrons of the target image, theanalyzer being orientated such that the electrons are dispersed in adirection substantially perpendicular to the length of the target image,whereby the analyzer produces a rectangular image made up ofsubstantially parallel strips each including electrons of a range ofenergies but each resulting from the scattering by the target ofelectrons of a respective energy.
 2. An electron spectrometer accordingto claim 1, wherein the electron selector and analyzer each comprise ahemispherical deflector.
 3. An electron spectrometer according to claim1, comprising:three electron lenses positioned in series between theselector and target image positions, a first electron lens beingarranged to focus the selector image on a slit orientated to pass a beamof electrons having a range of energies, a second electron lens beingarranged to focus electrons passing through the slit onto the target,and a third electron lens being arranged to focus electrons scatteredfrom the target to form the target image.
 4. An electron spectrometeraccording to claim 3, comprising:a pair of resistive plates arranged onopposite sides of the electron beam between the slit and the secondelectron lens, and means for passing equal and opposite currents throughthe plate to rotate the electron beam about the axis of the electronlenses.
 5. A method for producing a two-dimensional image of a target inan electron spectrometer, said method comprising:generating a beam ofelectrons having a range of energies dispersed in energy to produce anelongate selector image each portion of the length of which includeselectrons having a predetermined respective energy, the selector imagebeing focussed on the target to produce an elongate target imageincluding scattered electrons having a range of energies, each portionof the length of the target image resulting from a respective portion ofthe length of the selector image, and the scattered electrons of thetarget image being dispersed in energy, the direction of dispersion ofthe scattered electrons being substantially perpendicular to the lengthof the elongate target image, whereby a rectangular image is formed madeup of substantially parallel strips each including electrons of a rangeof energies but each resulting from the scattering by the target ofelectrons of a respective energy.